Light in the near- to mid-infrared region corresponds to an absorption spectrum region related to biological objects such as plants and animals and the environment. Therefore, development of a photodetector in this wavelength region has been conducted. Light receiving elements that include a light-receiving layer composed of a group III-V compound semiconductor have become mainstream. For example, a known example of a detector includes a light receiving element array having a responsivity that is extended to a wavelength of 2.6 μm by using extended-InGaAs as a light-receiving layer (NPL 1). In the light receiving element array, InAsP that is lattice-matched with the InGaAs light-receiving layer is used as a window layer.
A light receiving element including a GaSb substrate and a light-receiving layer provided on the GaSb substrate, the light-receiving layer having a type-II (InAs/GaSb) multiple quantum well structure, has also been proposed (NPL 2). It is described that this light receiving element has a responsivity up to a wavelength of close to 12 μm.
A light receiving element having an n-B-n (n-type layer/barrier layer/n-type layer) structure has also been proposed in which a light-receiving layer having a type-II (InAs/GaSb) multiple quantum well structure is provided on a GaSb substrate and a barrier layer is disposed in the middle of the light-receiving layer (NPL 3). As compared with a light receiving element having a p-i-n structure, a light receiving element having the n-B-n structure has the following advantage: Since diffusion of a hole is used for detecting light, the depth of mesa etching for pixel isolation can be made shallow and a noise current flowing in a sidewall of a mesa structure can be reduced.